Laterality and Visual Perception in Monkeys

Author: Vermeire, Betty Anne

Year: 1980

Degree: Dissertation (Ph.D.)

Advisor: Hamilton, Charles Robert

Committee Member: Unknown, Unknown

Option: Biology

DOI: 10.7907/yvpr-a497

Abstract

Split-brain rhesus monkeys were tested for differences in the abilities of their two cerebral hemispheres to process visual stimuli. Tests of visual preferences, i.e., what each hemisphere likes to look at, utilized colored photographs as stimuli and in most cases revealed an overall greater preference by the left hemisphere. Furthermore, for some types of stimuli, the hemisphere with the greater preference was correlated with each monkey's handedness; the hemisphere ipsilateral to the preferred hand, possibly equivalent to the human non-dominant hemisphere, showed greater preferences. The monkeys' ability to make same-different judgments about two sequentially-presented visual stimuli was also assessed. The hemisphere contralateral to the preferred hand, possibly equivalent to the human dominant hemisphere, learned this task faster. These results and other examples of left-right differences and of correlations of hemi-sphericity with handedness in rats, monkeys, and humans reported in the literature suggest that the basis for hemispheric specialization, particularly in infrahuman mammals, may be quantitative asymmetries in emotional and/or attentional processes.

A separate series of experiments investigated the role of selective attention in mirror image discrimination. Normal and partially split-brain monkeys learned both up-down and left-right mirror image discriminations with the place of response in different problems either aligned with the axis of symmetry of the patterns, i.e., close to the salient cues, or 90° orthogonal. The relative ease with which the four possible combinations of mirror image orientation and place of response were learned by the different groups of monkeys, as well as the outcome of tests of interocular transfer and generalization to a new place of response, indicated that restricting attention in various ways can influence the discriminability of mirror images. Theories based on the anatomical connections of the brain are insufficient to account for all of the results of this experiment. Instead, the results support the hypothesis that behavioral asymmetries in attention may account for the difficulties normal animals and people have in telling left from right.

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